Hello, I'm Vladislav. For my summer holidays, I usually stay in a country house near Moscow.
      Last summer, my dad's brother gave me a present — a blue bike. At that time I couldn't ride it very well. One day I got into an accident. But don't worry, everything was OK, just some “little” injuries.That day, I tried to ride my bike and suddenly it began to move very fast. I couldn't stop the bike. So I fell into a ditch (沟) full of dirty water. When I thought I had had enough bad luck, a girl walked by. She saw the bike and me. I quickly stood up.
   “Are you fishing here?” she asked.
   “Yes,” I answered. Then I said, “I'm making fish soup for dinner. Would you like to come?” To my surprise, she agreed.
      After that we became friends. Now, we often send each other e-mails. She knows that I’m studying in China and she asks me questions about Chinese culture. I tell her my stories of China, and she says she wants to visit China, too. She even plans to study Chinese. This is how, with the help of my bike, China got one more Russian friend.
根据短文内容，回答下列问题。

某同学用如图所示滑轮组将重为300N的物体以0.2m/s的速度匀速向上拉动，拉力为125N（不计绳重及摩擦），则动滑轮重　 ____N，5s内人拉绳子做的功是　 ____J，该滑轮组的机械效率是 ____

当电磁波的频率减小时， 它在真空中的速度将(    )